The invention involves a method to determine a physical quantity by means of acoustic transit time measurement using at least one acoustic transducer as a transmitter and at least one acoustic transducer as a receiver. The invention also includes an apparatus adapted to implement this method.
Such a method and such an apparatus have been described in WO93/0057. The apparatus described there uses acoustic transit time measurement to determine the temperature of a turbulent gas flow through a measuring space. It reduces the effects of noise on the measuring signal by using transmitted signals of a specific, selected frequency and aims to achieve a relatively accurate transit time measurement by means of the corresponding correlation operation. This method, however, is not sufficiently accurate for certain applications.
The present invention attempts, among other things, to overcome this limitation. For this purpose, a method of the type referred to above is characterised by a transit time measurement using an acoustic transmitted signal which is at least partly selected on the basis of the type of signal processing to be used, and at least two points in an acoustic received signal and/or a derived signal, which are also selected at least partly on the basis of the type of signal processing to be used.
Signal processing in order to determine the quantity to be measured involves determining the point in time at which the acoustic signal received shows the expected signal shape. This is achieved by applying a detection method to at least two specific points in the received signal. These points in the signal are representative of the shape of the relevant part of the acoustic received signal.
Since a known transmitted signal is used, and the transfer functions of the transducers and the medium are highly predictable, the detection system can make use of a relatively high level of prior knowledge of the expected received signal. This property, combined with the utilisation in the present invention of at least two points selected at least partly on the basis of the type of signal processing to be used in a received signal which is also selected at least partly on the basis of the type of signal processing to be used, allows considerable improvement of the accuracy of detection and hence of the accuracy of the transit time measurement.
The transmitted signal to be used is preferably selected at least partly on the basis of the geometry, the type of medium and other characteristics and features of the measuring space. In most cases, however, these characteristics solely, or at least largely, determine the frequency range of the transmitted signal, rather than, e.g., the frequency course of the transmitted signal, which means that this frequency course can be set to optimise the results. The accuracy of the transit time measurement can be improved by striving towards a large information content in the transmitted signal, the information content being adapted to the measurement path, the medium and the transducers. As a result of reflections, a known transmitted signal is received by the receiving transducer at the other end as a compound signal, which repeatedly includes the transmitted signal, whether or not distorted.
It should be noted that the transit time of an acoustic signal in a medium over a particular path can only be defined for a monochromatic signal and is then only valid for a homogeneous and isotropic medium in a stationary condition. In a compound signal, each frequency component has its own transit time for a particular path, since the frequency dependence of the velocity of sound results in velocity dispersion. Therefore, using a compound signal necessitates the use of weighted transit times.
In measuring particular physical quantities, such as temperature or distance, weighted transit times can be determined by means of algorithms which use information about the transmitted and detected signals and prior knowledge of other parameters. The functionality of the signal interpretation can be realised by means of algorithms based. on operations such as filtering, correlating, interpolating, curve-fitting, statistical data processing and analysis in a time-frequency domain. Signal interpretation may result in a comparison function which yields, for each point in time, a measure of the similarity between the received signal and the transmitted signal or a signal derived from the latter.
In one preferred embodiment a weighted transit time of an acoustic signal is determined using knowledge consisting of models for the medium and/or the measuring space about the distortion which the signal undergoes on its path through the measuring space. This embodiment preferably uses an aperiodic transmitted signal, of limited duration, in order to reduce the influence of spatial reflections on the signal received. This allows highly accurate weighted transit time measurements for long measurement paths.
The knowledge required for signal generation and interpretation can be divided into initial knowledge, i.e., characteristics and features introduced into the system, and knowledge which changes during the process of measuring. The initial knowledge includes information on system data and system behaviour, parameterised models of the medium and the measuring space, and information on processes taking place in the medium. This initial knowledge is adapted on the basis of the changing process and environmental conditions, including changes in the measuring space, the measurement path and process conditions such as flow.
A further preferred embodiment is characterised by the fact that an acoustic signal, emitted by a transmitter, is acoustically reflected one or more times through the measuring space before being received by the receiver, which may or may not be the same as the transmitter. Thus, the transmitter and the receiver may be one and the same transducer. The transmitted signal is reflected through the measuring space by means of acoustic mirrors before being received by the same transducer, which now functions as a receiver. In addition to the advantage of requiring only one transducer, the use of reflection results in a fairly long measurement path even within a relatively small measuring space. This raises the accuracy of the local acoustic transit time measurement and reduces the influence of external factors such as the environmental temperature on the measuring system and hence on the transit time measurement, provided a suitable construction is used for the measuring system and the housing of the measuring space.
In a further preferred embodiment, the two points selected in a received signal are taken from the slope and the top of the signal, and are used for signal shape detection with the help of specially adapted signal processing based on combined slope and top detection of a received acoustic signal. If, for instance, analogue signal processing uses combined slope and top detection of a selected acoustic signal transmitted by a transmitting transducer, then the use of a transmitted signal selected on the basis of the in this case analogue signal processing technique and the two points in the received signal mentioned above allow detection with a relatively high level of selectivity and prior knowledge of the measuring signal. This results in a simple and extremely sensitive detection technique, unlike conventional signal level detection, in which a received signal, attenuated by damping, is detected at a different geometric part of the signal slope received. A well-known improvement of this latter technique is to repeat the measurement using adjusted amplification levels. The combined slope and top detection system applied in the present invention aims at shape detection based on knowledge of the signal shape, using slope detection as an indication of the expected signal top. The actual shape detection is implemented by determining the position of the signal top corresponding to the detected signal slope. Detection on the basis of the position of the signal top is quicker and invariant over time as regards signal attenuation.
Another preferred embodiment involves determination of the transit time of an acoustic signal by means of a method for calculating the time-dependent similarity between the shapes of two signals, which are derived whether or not according to models for the medium and measuring space from the transmitted and the received signal respectively. In particular, use is made of time-discrete calculation methods, based on a discrete model of the time-dependent similarity between the signals, using regression and numerical approximation methods to overcome the limitations of time-discrete calculation methods.
A further preferred embodiment uses a low-frequency transmitted signal to avoid loss of information within the acoustic path as a result of excessive damping over a long measurement path and/or reflections from geometric obstructions which are small relative to the wavelength of the acoustic signal used. The main advantage of the use of a low-frequency transmitted signal for long measurement paths is the reduced damping, as well as the fact that the signal, because of its larger wavelength, is not distorted by relatively small geometric obstructions in the acoustic path. In addition, the use of a transmitted signal with known frequency characteristics allows reduction of the measuring errors introduced by acoustic signal distortion by reflections and non-stationary phenomena within the measuring space and by Doppler-like phenomena resulting from transducer movement and/or inhomogeneous and/or non-stationary medium flow. The type of transmitted signal to be used is at least partly determined by the transfer functions in the signal path, including those of signal processors, the transducers and the medium to be investigated, as well as by obstructions in the measuring space and the geometry and other characteristics and features of the measuring space. This ensures that transmitted and received signals and signal processing are ideally suited to each other and allows the transmitted signal to be adapted to changing environmental and process conditions, thus improving the quality of data processing. Using the shape features of the transmitted signal with its known frequency characteristics makes it easy to take account of the signal s frequency distortion between transmitter and receiver. As was noted above, low-frequency signals with a relatively large information content are used, if possible, for longer measurement paths. This preferably involves the use of a relatively short sweep signal, i.e., a signal with a known, time-dependent, variable phase velocity.
Signals with a large power content and known information content are highly suitable for correlation techniques and can yield highly accurate results in real time from a single measurement, preferably using weighted correlation to measure transit time. A weighted correlation algorithm is employed to make optimum use of the information content of the received signal, while taking the transfer functions of the medium and the transducers into account. The acoustic transit time is determined with the help of correlation techniques by performing a cross-correlation between the transmitted signal or a signal derived from the latter and the received signal, with the position of the cross correlation top constituting a measure of the acoustic transit time.
In a further preferred embodiment, the resolution of the correlation operation determining the transit time of the acoustic signal is improved with the help of numerical approximation methods and curve-fitting, using a relevant, approximated section of the autocorrelation function of the transmitted signal or a signal derived from the latter based on prior knowledge of the shape of the correlation function, in order to determine accurately the position of the cross correlation top. A discrete transmission signal is derived from a predefined transmission signal, e.g. a sweep signal. In addition, a reference signal is defined as a discrete model of the signal at the receiving end, whose shape is as similar as possible to that of the measuring signal to be received, for the purpose of signal comparison. On the basis of this, a discrete autocorrelation of the reference signal therefore yields a function which can be assumed to be identical in shape to the cross correlation function obtained from the reference signal and the received measuring signal.
For the purpose of detection signal processing, the discrete autocorrelation function of the reference signal is directly fitted to the cross-correlation function by means of regression. The model is refined by oversampling the autocorrelation function of the reference signal and/or is described by a contour of the autocorrelation function, which is described by a Curve-Fitting model (C-F model). The C-F model is determined for a relevant portion of the function, which includes the correlation top. For this purpose, a continuous mapping is determined, interpolating the discrete correlation values. Thus, it constitutes a partial model of the correlation function which can be calculated a priori. The position of the top within the contour can be determined in advance by analytical or numerical means, after which it is available as a parameter value. This method offers much higher accuracy and resolution, as well as greater speed, than conventional signal processing based on correlation calculations and statistical data processing.
In practice, detection signal processing is achieved by calculating the cross correlation function using sample values determined at a relatively low sampling frequency. This means that the corresponding discrete correlation values are relatively far apart in time, precluding accurate top detection in the correlation function. The abovementioned contour of the C-F model, in which the position of the top is known, is used to fit the model to the calculated discrete correlation values by means of weighted regression analysis. This allows the ultimate position of the C-F model, positioned over the correlation values, to be determined with a high level of resolution, ensuring accurate determination of the position of the cross-correlation top. The C-F model, the parameter value for the position of the top in the model and the weight factors for the regression analysis are stored in the system as embedded knowledge.
In a further preferred embodiment, weighted transit times are determined accurately by means of a predefined signal distortion over a given measurement path, using models for the medium and the measuring space. The transmitted signal and the medium and measuring space models are used to define a reference signal, which is then used to determine the time-dependent similarity between the reference signal and the received signal.
In a further preferred embodiment, the time-frequency dependent similarity between the transmitted and received signals is determined by means of analysis in the Wigner domain or another time-frequency domain. The advantage of the Wigner domain over other time-frequency domains is that the former imposes no fundamental restrictions on the time and frequency resolutions. Its disadvantage is that its use results in cross terms between the various signal components.
In a further preferred embodiment, cross terms are eliminated by using a transmitted signal such that the Wigner transform of the reference signal itself contains no cross terms. In addition, the transmitted signal is chosen in such a way that if the reference signal occurs repeatedly in the received signal, the resulting cross terms can be filtered out. This can be achieved, for instance, by summing multiple points of the Wigner transform. The comparison function is obtained by summing multiple points of the Wigner transform, with the time components of the latter points relative to the time for which the value of the comparison function is computed.
In a further preferred embodiment, a physical quantity is determined by measuring, by acoustic or conventional means, one or more other quantities in a space in at least partially identical circumstances. In this context, conventional means determining a particular quantity on the basis of a measuring method which is in common practical use. The other quantities can be determined in the same measuring space and/or in a different measuring space, under certain identical conditions. If the behaviour of the other quantity is sufficiently known, a measurement which is in fact relative can still yield an absolute value. Alternatively, a physical quantity can be determined by using the invention to determine another quantity with the same signal processing method but over a different measurement path. For instance, humidity in a particular measuring space can be determined by combining a transit time measurement in that space with a similar transit time measurement in a different space containing a medium of known composition.
In a further preferred embodiment, analysis and elimination of the influence on the signal processing of the interaction between the shortest-path signal and its reflections in the measuring space is achieved by using the medium and measuring space models as well as the fact that in a stationary measuring space, the shortest path and the reflected paths will remain roughly unchanged. In identical circumstances of medium and temperature, transit times for different paths will depend on the lengths of the paths only, allowing accurate adaptive and statistical determination of the transit times for the various signal paths, and hence also the weighted transit time for the shortest-path signal, in measurements over a longer period of time involving a temperature course, which may be cyclic or otherwise.
With the help of prior knowledge and the corresponding models of the medium and the measuring space, information on partially redundant acoustic measurement paths can be used to determine a particular quantity over a section of an acoustic measurement path. This allows the use of acoustic tomography, as described for instance in GB 2235294-A. This can be achieved not only with fixed transducers, but also with a system of transducers in linear or rotary motion, whether or not using one and the same transducer for transmission and reception.
One preferred embodiment of the invention is an apparatus fitted with an acoustic transducer for signal generation, an acoustic transducer for signal detection, a signal processing unit which selects at least two measurement points and a signal processing unit which processes a measuring signal transmitted by a transmission transducer. In particular, the apparatus is fitted with one or more acoustic mirrors which fold the measurement path.
A further embodiment of such an apparatus is fitted with a measuring-point selecting device which locates measuring points in a transmitted signal, coupled with a signal detection device allowing combined signal topxe2x80x94signal slope detection.